The present invention relates to a method for monitoring an SCR catalyst, in particular for monitoring the storage capacity of the SCR catalyst for ammonia.
The prior art includes methods and devices for operating an internal combustion engine, in particular on motor vehicles, in the exhaust zone of which an SCR catalyst (Selective Catalytic Reduction) is arranged that reduces the nitrogen oxides (NOx) contained in the exhaust gas from the internal combustion engine to nitrogen in the presence of a reducing agent. The proportion of nitrogen oxides in the exhaust gas can thereby be considerably reduced. To enable the reaction to take place, ammonia (NH3) is required, this being added to the exhaust gas.
Stricter laws in the area of diagnosis of components relevant to emissions require the monitoring of all exhaust gas aftertreatment components as part of On-Board Diagnosis (OBD) and also monitoring of the sensors used for compliance with OBD limits. The OBD limits are generally specified as a multiple of the legally stipulated emissions limits. When using an SCR catalyst, compliance with the OBD limit for nitrogen oxides must be ensured. The monitoring functions must ensure that overshooting of the corresponding limit due to aging of or damage to the SCR catalyst is reliably detected.
The reduction of the nitrogen oxide molecules from the exhaust gas takes place on the catalyst surface in the presence of ammonia as a reducing agent. The reducing agent is metered in in the form of an aqueous urea solution, which is injected upstream of the catalyst by means of a metering device. The desired metering rate is determined according to requirements in an electronic control unit, with the strategies for operation and monitoring of the SCR system generally being stored in the control unit.
SCR catalysts that are usually used store ammonia on the catalyst surface. Reduction of nitrogen oxides to elemental nitrogen, i.e. NOx conversion, in the SCR catalyst is all the more successful, the larger the amount of reducing agent available in the catalyst. As long as the storage capacity of the SCR catalyst is not yet exhausted, metered reducing agent which is not consumed for conversion is stored. If less reducing agent is made available by the metering unit than is required for conversion of the nitrogen oxides currently present in the exhaust gas, the ammonia stored on the catalyst surface is used, with the result that the NOx conversion taking place at the catalyst surface lowers the NH3 level of the catalyst.
SCR systems that are currently employed often have metering strategies which include level control of NH3 in the catalyst. With a level control system of this kind, an operating point is set in the form of a setpoint for the NH3 level in the SCR catalyst. This operating point is generally selected in such a way that the NH3 level is high enough, on the one hand, to ensure a high NOx conversion rate and a buffer for brief NOx spikes in the exhaust gas. On the other hand, the setpoint should be as far from the maximum storage capacity as is necessary to avoid ammonia slip through the catalyst. Such a breakthrough of ammonia occurs especially when the ammonia metered in is not used either to reduce the nitrogen oxides or to replenish the NH3 store, that is to say that the NH3 cannot be absorbed on the catalyst surface either. Since ammonia has a damaging effect on health and on the environment at high concentrations, breakthroughs of pure ammonia as NH3 slip should be avoided as far as possible.
Owing to the nature of the system, level control in the SCR catalyst, including the setting of the operating point for the NH3 level, is subject to large tolerances. This is due in part to the fact that there are currently no suitable measuring systems for directly measuring NH3 for use in a motor vehicle. On the contrary, use is generally made of nitrogen oxide sensors that have cross sensitivity for NH3, meaning that the sensor signal is a combined signal comprising NOx and NH3. Moreover, the reducing agent NH3 is not metered in directly but is generally made available in the form of an aqueous urea solution. This aqueous urea solution is converted into NH3 and CO2 in the exhaust system upstream of the SCR catalyst by thermal processes. The degree of conversion depends on many different factors and cannot be reliably estimated at all operating points. Finally, a nitrogen oxide sensor upstream of the SCR catalyst is often dispensed with in order to reduce costs, making it necessary to resort to a model value for the nitrogen oxide concentration currently prevailing in the exhaust gas, from which the desired metering rate is then determined.
An OBD-II-compliant SCR system has at least a nitrogen oxide sensor downstream of the SCR catalyst. As already mentioned, NOx sensors that are currently in common use indicate a combined signal comprising NOx and NH3. A rise in the sensor signal from a nitrogen oxide sensor arranged downstream of the SCR catalyst can therefore indicate either a falling NOx conversion rate, i.e. a rise in the NOx concentration, or a breakthrough of pure ammonia, i.e. a rise in the NH3 concentration. Direct discrimination between NOx and NH3 is not possible.
It is known that the NH3 storage capacity of an SCR catalyst is greatly reduced with progressive aging, especially due to thermal processes. The practice of using the NH3 storage capacity of an SCR catalyst as a diagnostic feature for monitoring the catalyst is therefore likewise already known. German Offenlegungsschrift DE 10 2007 040 439 A1, for example, describes a monitoring strategy for an SCR catalyst in which the NH3 storage capacity is determined and used as a feature for indicating the aging of or damage to the catalyst. In this strategy, the SCR catalyst is initially filled with reducing agent up to the maximum achievable NH3 storage capacity by superstoichiometric metering of the reducing agent in the form of overmetering, that is to say that the maximum quantity of NH3 is absorbed at the catalyst surface. As soon as the maximum storage capacity is reached, unbound NH3 breaks through the catalyst. Owing to the cross sensitivity of the nitrogen oxide sensor downstream of the catalyst to NH3, this NH3 slip is detected indirectly in the form of an increased sensor signal, which can be measured as an assumed dip in the NOx conversion rate. The maximum NH3 storage capacity that can be detected by means of the breakthrough of NH3 is used as a defined starting point for diagnosis. After the NH3 breakthrough has been detected, the metering of reducing agent is reduced relative to normal metering (undermetering) or completely shut down. During this process, the stored NH3 mass, i.e. the NH3 absorbed in the SCR catalyst, is gradually lowered again through use during the reduction of nitrogen oxides. During this “emptying test”, the SCR efficiency or other characteristic values dependent on the NOx conversion rate can be determined, and the usable NH3 storage capacity of the catalyst can be inferred indirectly therefrom.
There are various known versions of this monitoring strategy for avoiding the disadvantageous NH3 slip that occurs in the course of this monitoring strategy and for reducing the effect on exhaust gas aftertreatment of the metering carried out in the course of monitoring. One version is suitable particularly for SCR catalysts with a very high NH3 storage capacity when new and a greatly reduced NH3 storage capacity when aged. In this version, the overmetering phase is not always ended only after the detection of NH3 slip but as soon as an NH3 level in the SCR catalyst is reached, said level having been selected as a function of temperature. This selected NH3 level is specified in such a way that it lies between the maximum storage capacity of the catalyst when new and the maximum storage capacity of an aged catalyst. If this level can be reached without the occurrence of NH3 slip, it can be assumed that the catalyst is not yet aged to the extent that it must be considered faulty. The advantage with this version is that monitoring can be successfully ended without the occurrence of an effect on emissions in the form of NH3 (by NH3 slip during overmetering) or due to NOx (in the case of a low conversion rate in the emptying test). The disadvantage here is that only a catalyst that is as good as new can be detected by this method. An aged catalyst cannot be assessed during the overmetering phase.
In order to increase the accuracy of diagnosis in this version of the monitoring strategy, there is a known practice of metering in a defined quantity of NH3 selected as a function of temperature in the overmetering phase before making the transition to the emptying test. The effect is that, in the case of an aged SCR catalyst with a storage capacity that is too low for this quantity of NH3, the excess quantity of NH3 that is being metered in appears in the sensor signal of the nitrogen oxide sensor downstream of the SCR catalyst. In this case, the NOx conversion rate derived from this sensor signal is artificially lowered, enabling the NOx conversion rate in the overmetering phase to be used as an additional diagnostic feature. Owing to the tolerances in the system, however, it is not possible to ensure that the amount of excess NH3 metered in is always the same in the diagnosis of an aged catalyst since the calculation of the NH3 level during the overmetering phase is difficult. In practice, therefore, the emptying test must also be carried out in this version, despite the additional diagnostic feature in the overmetering phase, and this test disadvantageously leads to increased NOx emissions due to the falling NOx conversion rate.
Given this situation, it is the underlying object of the invention to improve accuracy in the monitoring of the storage capacity of an SCR catalyst for ammonia and furthermore to reduce the disadvantageous effect of conventional diagnostic methods on emissions of nitrogen oxides and/or of ammonia.